The installation of underground ducting and cabling is commonly accomplished through underground boring and pulling methods. In these methods, a pilot hole is typically drilled through the underground structure where the cable or duct is to be run. Once the pilot hole is completed, a drill steel is inserted back through the hole and a cutting head attached to the drill steel. The drill steel is then connected to a boring machine which rotates the cutter and pulls it back through the pilot hole thereby enlarging the hole to the required diameter. Alternately, pneumatic or hydraulic motors on the cutter may be used for rotational movement.
Typically a cable or duct is connected to the rear of the cutting head such that as the cutting head is pulled back through the pilot hole, not only is the pilot hole enlarged to the desired diameter but the cable or duct is pulled through simultaneously. Since the cutting head is rotating as the cable or duct is pulled through the freshly cut hole, a swivel is used between the cutter and the cable or duct to prevent rotation of the cable or duct. This method of both drilling a hole and installing a cable or duct simultaneously is often referred to as directional drilling. In some situations a two step process may be used where a hole is first drilled following which a cable or duct is pulled through.
Unfortunately, regardless of which of these methods are used, it is not possible to view the cable or duct as it is being pulled through the underground hole. It will be appreciated that the cable or ducting that is installed will have a maximum tensile load to which it may be subjected without causing structural damage. Accordingly, methods have been devised to monitor the tensile load applied to cables or ducts in such applications. Others have suggested monitoring the pulling force applied to the drill steel or fish line such that the load does not exceed the maximum safe tensile load of the cable or ducting being installed. While such methods may be somewhat useful when pulling a cable or conduit through a pre-existing hole, their use is severely limited where directional drilling is used. In cases where a cable or ducting is attached by way of a swivel to the back of a cutter head, the pulling force that must be applied to the cutter in most cases exceeds the maximum allowable tensile load for the cable or ducting. Monitoring the pulling force is therefore of little or no benefit.
Still others have proposed attaching folded over loops of wire, with a known breaking value, to the cable or ducting. When the looped wire breaks, the operator of the machinery knows that the load has been exceeded. For example, if looped wires having a breaking value of 500 pounds are to be used and it is decided that a safe maximum tensile load for the application is 1500 pounds, three looped wires would be used on the assumption that they would fail at or about 1500 pounds of tensile force. Obviously, this method of monitoring the load applied to a cable or ducting is fraught with problems, including difficulty of set up, overall time consumption, and the inability to accurately monitor precise loads and to duplicate pulls under various working conditions.